Hubble Detects Iron and Iron Oxide on Asteroid Psyche

Astronomers using the Space Telescope Imaging Spectrograph on the NASA/ESA Hubble Space Telescope have observed the main-belt asteroid (16) Psyche, the target object of NASA’s Discovery Mission Psyche, at ultraviolet (UV) wavelengths and found that the UV spectrum of the asteroid is best matched with the reflectance spectrum of pure iron; however, small grains of iron may dominate the spectrum even if this metal only comprises up to 10% of the material on the surface.

An artist’s concept of the asteroid Psyche, which lies in the main asteroid belt between Mars and Jupiter. Image credit: NASA / JPL-Caltech / ASU.

Psyche, a metal asteroid about 226 km (140 miles) in diameter, is one of the most intriguing targets in the main asteroid belt.

This asteroid orbits the Sun between the orbits of Mars and Jupiter at a distance ranging from 378 to 497 million km (235-309 million miles) from the Sun.

The object takes about five Earth years to complete one orbit of the Sun, but only a bit over four hours to rotate once on its axis.

Unlike most other asteroids that are rocky or icy bodies, planetary scientists think Psyche is comprised mostly of metallic iron and nickel similar to Earth’s core.

They wonder whether this asteroid could be the nickel-iron heart, or exposed core, of an early planet maybe as large as Mars that lost its rocky outer layers through violent collisions billions of years ago.

If so, it would provide a unique look into the Solar System’s distant past, when the kind of high-speed protoplanet encounters that created Earth and the other terrestrial planets were common.

“We’ve seen meteorites that are mostly metal, but Psyche could be unique in that it might be an asteroid that is totally made of iron and nickel,” said Dr. Tracy Becker, a planetary scientist at the Southwest Research Institute.

“Earth has a metal core, a mantle and crust. It’s possible that as a Psyche protoplanet was forming, it was struck by another object in our Solar System and lost its mantle and crust.”

Orientation of Psyche at the time of the two Hubble observations. Image credit: Becker et al., doi: 10.3847/PSJ/abb67e.

Dr. Becker and her colleagues from the United States and Sweden observed Psyche at two specific points in its rotation to view both sides of the asteroid completely and delineate as much as possible from observing the surface at UV wavelengths.

“We were able to identify for the first time on any asteroid what we think are iron oxide ultraviolet absorption bands,” Dr. Becker said.

“This is an indication that oxidation is happening on the asteroid, which could be a result of the solar wind hitting the surface.”

The researchers also observed that the asteroid’s surface could be mostly iron, but they noted that the presence of even a small amount of iron could dominate UV observations.

However, while observing Psyche, the asteroid appeared increasingly reflective at deeper UV wavelengths.

“This is something that we need to study further,” Dr. Becker said.

“This could be indicative of it being exposed in space for so long. This type of UV brightening is often attributed to space weathering.”

The results were published in the Planetary Science Journal.


Tracy M. Becker et al. 2020. HST UV Observations of Asteroid (16) Psyche. Planet. Sci. J 1, 53; doi: 10.3847/PSJ/abb67e

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